This invention relates generally to semiconductor processing and, more particularly, to an improved gas distribution system, for instance, for a chemical vapor deposition chamber to provide improved transient phase deposition.
One of the primary steps in the fabrication of modem semiconductor devices is the formation of a thin layer on a semiconductor substrate by chemical reaction of gases. Such a deposition process is referred to generally as chemical-vapor deposition (“CVD”). Conventional thermal CVD processes supply reactive gases to the substrate surface where heat-induced chemical reactions take place to produce a desired layer. Plasma-enhanced CVD (“PECVD”) techniques, on the other hand, promote excitation and/or dissociation of the reactant gases by the application of radio-frequency (“RF”) energy to a reaction zone near the substrate surface, thereby creating a plasma. The high reactivity of the species in the plasma reduces the energy required for a chemical reaction to take place, and thus lowers the temperature required for such CVD processes as compared to conventional thermal CVD processes. These advantages are further exploited by high-density-plasma (“HDP”) CVD techniques, in which a dense plasma is formed at low vacuum pressures so that the plasma species are even more reactive. “High-density” is understood in this context to mean having an ion density that is equal to or exceeds 1011 ions/cm3.
Particular applications that lend themselves to effective use of HDP-CVD techniques include shallow-trench isolation (“STI”), premetal dielectric (“PMD”) applications, and intermetal dielectric (“IMD”) applications. One issue that affects deposition properties in various such applications is diffusion between adjoining layers that have different compositions, which can adversely affect certain desired properties of the resulting layer structure. One approach that has been used to prevent such diffusion includes deposition of an additional intermediate barrier layer. For example, when doped silicon oxide is deposited in IMD applications, diffusion of the dopant to metal lines may cause the formation of undesirable chemical species at the oxide/metal interface, resulting in poor adhesion between the oxide and the metal. Deposition of a silicon-rich liner on the metal prior to depositing the doped silicon oxide layer acts to prevent dopant diffusion. Including the barrier layer has the beneficial effect of improving adhesion in the structure. It is almost routine now in many applications to deposit barrier layers when forming certain structures. For example, a silicon-rich oxide liner is commonly formed on a substrate prior to deposition of a layer of fluorine-doped silicon oxide in fluorosilicate-glass (“FSG”) applications using HDP-CVD.
The deposition of an initial deposition layer or liner is a key component in preventing plasma damage in HDP-CVD reactors. There is substantial difficulty in achieving a uniform liner due to the nonuniform gas distribution in the transient phase of initial deposition. One current approach to deposit a uniform liner employs a low pressure strike which involves gas mixing in the chamber without plasma. During the mixing step, the substrate is cooling without the plasma, thereby lowering the deposition temperature of the liner. The liner precursor gases typically may include oxygen and a silicon-source gas such as silane, and perhaps also a fluorine-containing gas such as SiF4. Striking of the plasma after the premixing step may proceed by a low-pressure strike such as described in the copending, commonly assigned U.S. patent application Ser. No. 09/470,819, filed Dec. 23, 1999, entitled “LOW PRESSURE STRIKE IN HDP-CVD CHAMBER.” Use of low pressure strike also avoids plasma instability during the plasma stage ignition period, which might otherwise contribute to inconsistent film quality.
On the other hand, maximizing the deposition temperature has been demonstrated to be a key gapfill component in a HDP-CVD reactor. By lowering the deposition temperature using low pressure strike, the gapfill characteristics will tend to suffer.